Method of operating a nuclear reactor



Nov.v 3, 1964 w. A. FREDERICK METHOD OF OPERATING A NUCLEAR REACTOR 5Sheets-Sheet 1 Filed Dec. 20, 1957 INVENTOR William A. FrederickWITNESSES AT RNEY Nov. 3, 1964 w. A. FREDERICK METHOD OF OPERATING ANUCLEAR REACTOR 5 Sheets-Sheet 5 Filed Dec. 20, 1957 62 6 O 8 a 4L 0 m f2 2 4 6 .M l 4 m M w u I 6 I 3 m 0 6 l 4 E m 2 M IA 3 .m 25:; ,3 6 fiV J\4 6 M B m 6 DOD 3 M 9 I ll 3 5 w 2 3 I 8 c "PM r E-Rom 2 2 W. 5 & m 0 gM 2 4 i m M m .M .I- Q 2 7| T .I F m 59.0 2 m I 4 S .m O m 2 B .m ,n m3:12am mm H mm m i r c k 4 8 c I D a m n r 8 2355225 1+ 5 on r 7 87 2 4m o J E m 2 05: 6 m l rm l 5 u 3 w m a T m 3 IL 8 2 6 l .w I 0 R 0 3 4 9r l 8 u n n. 6 D o 3 I 5 J Q Q 3 c c 8 R V S hotzcaom 8 A 2 5 9 5 6 w IL, 2 oE S m 8 4 w 0 O 6 .p l v I C r 9 095m 9 W M. l 2 6 l 4 04. k 8 WmH 8 '6 23 3 r 2 c m 2 l m B W o p A 5... 1 r 8 b O m m d 2 4 005m 0 4 6M W rn3 I .lll/ o R 2 5 m n 2 4 4 8 Z 4 3 mm 7 5 w w M f 2 PEQEm v f S.m 8 9 5 6 M W I- 8 4 n I e a .z I l 0 u R J T I IL. W n S 0 Mm 3 m M 4Q I I 6 7 m a b w D m W 8 m 3 6 n 0 m 4 I. I I l l I: N 3225:; mm 2 M 9W U w I E M a b 5 C V...- o 8 9 8 It 7. o H O 4 a 6 9 O 5 u 6 7 I u m 22 m m Q. R s w 2 M a II 5 II\ I Nov. 3, 1964 W. A. FREDERICK METHOD OFOPERATING A NUCLEAR REACTOR Filed Dec. 20, 1957 Fig.2C.

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C O .1 OJ- 0 i 2 i E i 0 IO 20 3O 4O 50 Time (Hours) Change in XenonPoisoning After Shutdown 5 Vessel Outlet Manlfold O 550 348 22 .2 00":Average 900 a "-800 g 500 350 Vessel Inlet Manifold g g -soo 3 mGenerator OuHet 500 2 P m 450 352 Fraction of Full Power United StatesPatent 3,1555% METHUD F @PERATENG A NUCLEAR REACTUR The presentinvention relates to a method for operating a neutronic reactor, andmore particularly to a method of operating a variable-fuel type reactor.

In certain types of neutronic reactors, an example of which is describedhereinafter in greater detail, the reactor and the primary circulatingsystems associated therewith are arranged for supplying nuclear fuel orfissile material to the reactor in varying masses or concentrationrelative to the volume comprised within the reactional vessel. Thequantity of fuel contained within the reactional vessel may be varied ina number of ways. For example, a number of fuel rods containing fissilematerial can be inserted into or withdrawn from the reactional vessel inthe manner in which the control rods of certain types of reactors aremoved relative to the vessel, or the fissile material might be conveyedin varying quantitles to the reactor vessel in a form of a fluidizedsolid or in a form of larger discreet particles either of which may besupported by a gas or other vehicular fluid. In still other neutronicreactor systems the fissile material can be supplied to the reactionalvessel in the forms of a metallo-organic compound of the fissilematerial which is a liquid at reactor operating temperatures, a solutionof a soluble fissile material such as uranyl-sulfate, or as a suspensionor slurry of desirably ultra fine particles of the fissile material ortheir oxides in a suitable vehicle such as that described hereinafter.Although the invention disclosed herein is adapted for use in operatingany of the aforementioned variable concentration-type reactors, theinvention will be described in greater detail in connection with aquasi-homogeneous, slurry-type reactor.

It has been found that, in those neutronic reactors wherein theconcentration of the fuel can be varied readily, there are twoconcentrations of the fuel whereat the neutronic reactor can maintaincritically (Keff 1) at substantially the same operating temperatures.These concentrations are designated respectively as the high and lowcritical concentrations or points. However, in the range of fuelconcentrations between the critical concentrations, the reactor wouldbecome supercritical (K l) relative to the desired operatingtemperature, and the temperature would increase markedly, if no meanswere employed to otherwise control the reactor. It is desirable,however, to operate the neutronic reactor at the high criticalconcentration for the reason that, with the added fissile materialthereby included within the reactor vessel, a greater neutronic economyis obtained and hence a greater conversion ratio of fertile isotope,usually included in the fissile material, to one of the fissionableisotopes is likewise secured. it has likewise been found that whenoperating an efficient neutronic reactor at the high criticalconcentration that conversion ratios of unity or greater can beobtained, that is to say, that at least as much fissionable isotope istransmitted from the fertile material as is consumed in the chainreaction sustained in the initial supply of fissiona'ole isotope. Thebasic mechanisms whereby the fertile isotopes are converted into thecorresponding fissionable isotopes in a neutronic reactor are describedhereinafter in greater detail.

As pointed out previously, the reactor temperatures corresponding to theaforementioned high and low critical Bhldfifidd Patented Nov. 3, 1964concentrations are substantially the same. However, the averagetemperature of the reactor rises to a high peak between these two pointsas the concentration is raised from the low critical point to the highcritical point if all other factors remain the same. This conditionresults in the aforementioned supercritical condition wherein theefiective coefiicient of cfiticality (K becomes slightly greater thanunity at concentrations between these critical points. The temperatureupswing in certain cases is sufficiently high, if uncontrolled, toexceed the design limitations of the reactor and the associated primaryequipment.

In spite of the foregoing remarks, it has been found that the safestmethod of adding fissile material to the reactor is to fill the primaryreactor system initially with either a dilute mixture of fissilematerial and a suitable vehicle or with the vehicle alone. The dilutemixture, if employed, is maintained at a concentration which issufiiciently low to preclude criticality under any conditions. Theconcentration of fissile material then is gradually increased throughthe low critical concentration to the high critical concentration oroperating concentration. As the concentration is increased between thelow and high concentrations suitable means, certain forms of which aredisclosed and claimed hereinafter, are employed to control the reactorin this area of supercriticality.

It may be suggested that the reactor system be filled initially withfissile material at a concentration above the high criticalconcentration, which is then gradually reduced to the operatingconcentration. This method is not feasible due to the possibility of aportion of fissile material settling out of the vehicle after additionto the reactor, particularly before the circulating pumps can bestarted, and to the attendant great danger of unpredictable criticalityin the fissile material when thus diluted. Moreover, the fissilematerial would have to be maintained at an increasingly high temperatureas the concentration is reduced in order to preclude prematurecriticality. For practical purposes, then, the reactor system would haveto be raised to operating temperature by means of an external source ofheat before beginning to decrease the concentration of fissile materialin order to preclude the possibility of thermal shock.

It may be suggested also that the primary reactor system be filledinitially with fissile material at the operating or high criticalconcentration. This method likewise is inappropriate due tounpredictable reactivity of the fissile material while being added inthis concentration. Both the reactor system and fissile material wouldhave to be preheated in some fashion to a temperature in excess ofoperating temperature to prevent premature criticality and thermalstresses in the reactor system. Moreover, when employing certain typesof vehicles for the fissile material, these vehicles must be maintainedunder considerable pressures to prevent boiling at the operating reactortemperatures. However, the required pressurizing is impractical untilthe reactor system, including usually a reactional vessel and a numberof cir culating or cooling loops, is completely filled. Furthermore, theprimary circulating pumps cannot be started until the reactor system iscompletely filled, if the fissile material is in liquid form, because ofvapor binding. Consequently, a portion of the fissile material maysettle out of the vehicle, whereupon the concentration of the remainingfissile material may fall into the aforementioned super-critical areabetween the low and high critical concentrations.

Heretofore it has been the practice to limit the temperature peaks inthe aforementioned reactor types, which usually do not employ controlrods, by adding a socalled reactor poison to the fissile material. Thereactor poison is a material having a high neutronic capturecross-section, particularly for thermal neutrons, which are mosteflicient in causing fission in atoms of the fissionable isotope. Theexamples of these materials are xenon 135 having a neutronic capturecross-section of 3.2 barns for thermal neutrons and boron 10 and cadmium113 having cross-sections of 27,000 and 4020 barns, respectively. Thereactor poison has the effect of temporarily increasing the fuelconcentration at which the reaction will approach cri-ticality until thehigh critical concentration is reached, thereby effectively bypassingthe low critical concentration of the reactor system. However, in thismethod of starting up the reactor the concentration of both the fissilematerial and the reactor poison must be carefully controlled in orderthat the reactor will approach criticality at the normal high criticalconcentration of the fissile material. Such control is essentialinasmuch as insufficient power will be generated by the reactor shouldthe concentration of the fissile material exceed the high criticalconcentration. This method of operating the reactor suffers from thefurther disadvantage in that complicated auxiliary equipment is requiredto extract the reactor poison before full power operation can beattained, or in that the reactor must be operated at low power once thehigh critical concentration is obtained in order to burn out the reactorpoison as by converting the high cross-section isotope to a differentisotope having a low or negligible thermal neutronic capturecross-section.

It has also been proposed to permit the temperature to rise to theaforesaid high peak, which temperature increase limits the reactivity ofthe nuclear fuel contained within the reactor by means of the phenomenonresulting in the negative temperature coefiicient of reactivitydescribed hereinafter. This mode of operation requires added designstrength in the reactional vessel and as sociated components in order towithstand the pressures required to prevent the nuclear fuel orcomponents thereof from boiling at the highest anticipated temperatures.

In view of the foregoing, it is an object of the invention to provide anovel and eflicient method for operating a neutronic reactor,particularly during start-up and shutdown thereof.

Another object of the invention is to provide a novel method foroperating a neutronic reactor which does not require the addition of anexternal reactor poison.

A further object of the invention is to provide a novel method forstarting up a variable concentration type neutronic reactor having highand low critical fuel concentration characteristics.

A still further object of the invention is to provide a novel method forincreasing the fuel concentration of a neutronic reactor from a lowcritical concentration to a high critical concentration withoutsubstantially increasing the operating temperature thereof.

Yet another object of the invention is the provision of a novel methodfor shutting down a variable concentration type neutronic reactor havinghigh and low critical fuel concentration characteristics.

These and other objects, features and advantages of the invention willbe made apparent during the forthcoming description of an illustrativeembodiment of the invention with the description being taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic and elevational view, partially sectioned, of anexemplary homogeneous-type reactor vessel shown in conjunction withprimary coolant loop circuitry;

FIGS. 2A, 2B, 2C constitute a schematic fluid circuit diagram of ahomogeneous reactor system such as that illustrated inFIG. 1 andarranged for use with certain auxiliary equipment;

FIG. 3 is a graph showing a series of temperature curves of reactivityplotted against slurry concentration;

FIG. 4 is a graphical representation of the production and a highneutronic scattering cross-section.

materials for these purposes include carbon and the veof xenon isotopewithin the reactor system as a function of time after shut-down andexpressed as a fraction of the total parasitic neutronic absorptionwithin the entire reactor system; and

FIG. 5 is a graphical representation of slurry and steam temperature ofthe reactor system described herein, as functions of reactor poweroutput.

Generally speaking, in a homogeneous-type reactor system, the nuclearfuel is contained within the system as a liquid or suspension which insome cases may be a liquid compound. of at least one of the fissileisotopes noted below. In other cases, the liquid fuel comprises asuspension in a suitable vehicle of a pulverulent form of one or more ofthese fissionable and fertile isotopes, .or.

combination thereof, or a solution of one or more compounds thereof in asuitable solvent such as water. In those systems wherein the fuel isemployed as a conventional suspension or slurry or in someotherfiuidized form wherein the fuel is present in discrete, movableparticles, the reactor system is sometimes designated asquasihomogeneous. As explained more thoroughly hereinafter, thefluidized fuel is circulated through a reactor vessel by one or moreprimary circulating loops provided with suitable pumping means. Thefluid fuel including the vehicle or solvent, which usually serves bothas coolant and moderator, thus circulates through both the vessel andthe circulating loops in contradistinction to a heterogeneous typereactor system. In the latter class of reactors the fuel, moderator, andthe coolant or coolant-moderator usually are physically separated and atleast the fuel is mounted fixedly and entirely within the reactionalvessel.

The homogeneous reactional vessel is fabricated of such size and shapethat a quantity of the circulating fluid fuel contained therein isequivalent to the critical mass of the chain-reacting isotope includedin the fuel and conse quently a self-sustaining chain reaction can beestablished in the vessel. In the case of a quasi-homogeneous reactor,the concentration of the fissionable or chain-reacting isotope in theslurry or suspension can be adjusted within rather wide limits such thatthe aforesaid size and shape of the vessel can be varied accordingly asdesired. As pointed out hereinafter, the remaining components of thesystem are insuflicient in size and are suitably spaced or shielded suchthat a critical mass cannot be accumulated elsewhere in the reactorsystem. The heat developed Within the circulating fuel as a result ofthe nuclear chain reaction is removed from the fuel as it circulatesthrough the primary loops by suitable heat exchanging means coupledwithin each of these loops.

The vehicle or solvent employed with the circulating fuel, which may beordinary water (H O), heavy water (D 0) or an organic material havingthe desired characteristics of temperature and radiation stability,serves as a moderator for the chain reaction in addition to serving as aheat transfer medium as noted heretofore. As is well known, a moderatormaterial usually is employed adjacent the nuclear fuel to slow the fastneutrons produced by each fission to thermal velocity, whereat theneutrons are most etficient for inducing fission in atoms comprising thefissionable isotopes. More specifically, the moderator material slowsneutrons having energies in the neighborhood of ten million electronvolts to energies whichare equivalent to thermally excited hydrogenatoms or about 0.1 electron volt. As a result, the moderator materialappropriately is selected from a material having the characteristics oflow neutronic capture cross-section Suitable hicles or solvents notedheretofore, i.e., light and heavy water, and hydrocarbon organicmaterials which, of

course, contain carbon and hydrogen.

The homogeneous reactor system, presently to be described, is controlledinherently by the negative tempera 'ture coeflicient of reactivityassociated with the circulat-- -3 5.! ing nuclear fuel. This phenomenonis comparatively well known and is based upon the fact that an increasein temperature of the fuel contained within the reactor vessel decreasesthe density of both the fuel and the vehicular moderator and likewiseits moderating characteristics. By the same token, this decrease indensity increases the number of neutrons which are lost from theperiphery of the chain-reacting mass, and the resulting loss in neutroneconomy decreases the reactivity of the reactor system. Additionalcontrol is accomplished, as required, by diluting the circulating fuelwith additional vehicle or solvent, by adding a neutron absorbing poisonsuch as cadmium, boron, or xenon, or by draining the contents of thereactional vessel into a series of storage tanks presently to bedescribed. The latter arrangement also serves to terminate the chainreaction completely in an emergency or to shut down the reactor formaintenance and the like.

The fission products which are formed during operation of the reactordesirably are extracted continually from the system by means of chemicalprocessing in the case of solids, or in the case of gases, by means ofan offgas system such as that described in copending applications of D.Rinald and of I. Weisman et a1., filed October 21, 1957, Serial Nos.691,264 and 691,263, now Patents 3,080,307, dated March 5, 1963, and3,093,564, dated June 11, 1963, respectively, and both assigned to thepresent assignee. These fissional products cannot be permitted toaccumulate within the reactor system during normal operation thereofinasmuch as some of the daughter isotopes, particularly Xenon 135,quickly terminate or poison the chain reaction although present inrelatively small concentrations. As pointed out hereinafter in greaterdetail, use is made of this fact in controlling and operating a variablefuel concentration type reactor, in accordance with the presentinvention. In any event, the accumulation of these isotopes which resulteither directly, or indirectly through radioactive decay, from thefissional process would tend greatly to increase radioactivityassociated with the reactor plant, as compared to the conditionsobtaining were the fissional products continuously removed. As a result,the normal biological shielding requirements for the reactional vessel,the fuel circulating loops, and associated equipment would be greatlyincreased. Moreover, many of the fission-produced isotopes are valuableper se for those research, industrial, and medicinal applications, whichrequire high levels of the various radioactive emanations.

The circulating nuclear fuel in a simple burner type homogeneousreactor, contains a high percentage of one or more of the knownfissionable isotopes U U 1M amounting of course, to a quantitysufficient to sustain a chain reaction. Although a simple burner type ofreactor is relatively more elficient as to size and neutron economy, itsoverall fuel cost is very high. Moreover, substantially no additionalfuel is produced during operation of this type of nuclear reactor. Onthe other hand, in regenerative or breeder types of homogeneousreactors, an additional quantity of a fertile isotope such as 'lh or Uis mixed uniformly with the circulating fuel material. Thelatter-mentioned fertile isotope can be supplied in the form of naturalor source grade uranium which is primarily the U isotope containingapproximately 0.7% of U In a heterogenous type reactor, the samecombinations of fissionable and fertile isotopes can be employed, withthe exception that both groups of the fissile isotopes are fixedlymounted Within the reactor core and that the fertile isotope, commonlyreferred to as blanket material, usually surrounds the fissionablematerial. However, in a uniform, low enrichment heterogeneous reactor,several designs of which are either extant or under consideration, thesocalled blanket or fertile material, of course, is mixed uniformly withthe fissionable isotope. In the latter class of reactors, U usually isemployed which has been enriched to a greater than natural percentage ofU In an efficient reactor of the previously-mentioned regenerativetypes, it is possible to generate from the one or more fertile isotopesat least as much fissionable isotopes as is consumed in the chainreaction. If the conversional ratio is greater than unity, the reactoris classified in the breeder category.

During the progress of the chain reaction, each fissioned atom emits anaverage of two to three neutrons, some of which are classified as fastneutrons and must be slowed to thermal energies as noted previously.Approximately one of these neutrons is utilized in propagating the chainreaction. Another one of the neutrons is employed to initiate one of theseries of nuclear reactions described below, whereby an atom of thefertile or blanket material is transmuted into fissionable isotope whichmay be equivalent, for example, to the amount of fissionable materialconsumed in the chain reaction. If such is: the case, only the fertilematerial need be added to the reactor system during its operation. Theremainder of the fission-produced neutrons are absorbed in structuraland moderator materials, in nonfissioning capture by atoms of fissilematerial, and in peripheral escape from the chain-reacting mass.

Upon capturing one of the aforesaid fissional neutrons the fertilematerial U if employed, is converted into an isotope of the transuranicelement plutonium Pu in accordance with the following nuclear equations:

with the times denoted at the latter two reactions being the half-livesof the decaying isotopes. The transuranic isotope Pu which is one of theaforesaid fissionable isotopes, is endowed with a half-life of 24,000years and thus is relatively stable.

On the other hand, the artificial, fissionable isotope 92U233 isobtained when thorium 232 is employed as the fertile or blanketmaterial. The U isotope is formed as a result of the following series ofnuclear reactions:

The resultant fissionable isotope U having a half-life of 163,000 years,likewise is relatively stable.

Referring now more specifically to FIG. 1 of the drawings, an exemplaryvariable fuel concentration type of reactor system .is disclosed, whichis adapted for operation in accordance with the invention. In thisexample, the reaction system is a quasi-homogeneous, or slurrytypesystem, and comprises a reactional vessel 20 having a spheroidalconfiguration and provided :at diametrically opposite areas thereof withinlet and outlet man folds 22 and 24, respectively. The reactionalvessel 20 is of sufficient size to contain, as aforesaid, a criticalmass of the circulating nuclear fuel flowing through the vessel and theprimary loops of the reactor system. In this application, wherein acirculating slurry containing suspended, uniformly, admixed, pnlverulentoxides of thorium (T and highly enriched uranium U0 is employed, with avehicle including deuterium oxide or heavy water (D 0), the insidediameter of the instrument reactor vessel thermal shield 40 is of theorder of 13 feet. The aforementioned slurry, which is describedsubsequently in greater detail, thus includes a fissionable material inthe form of uranium 235 and a fertile material, thorium 232.Additionally, a small proportion of the fertile material, uranium 238,is included unavoidably with the U isotope.

In this example of the homogeneous reactor system, a total of fourcirculating loops, with only one loop 25 being shown in FIGS. 1, 2B and2C, are connected to the intake and outlet manifolds Z2 and 24 by meansof inlet and outlet conduits 26 and 28, respectively. The outlet conduit28 is connected to a gas separator 30 which in turn is coupled in serieswith a steam generating heat exchanger 32 coupled through a conduit 35to the suctional side 34 of a primary slurry pump 36. The gas separator30 is designed in a conventional manner and is arranged to removefissional and radiolytic gases from the system, which gasesare conductedout of the separator by means of a conduit 31. The steam generator 32which is provided, inter alia, with a feed water inlet 33 and a steamoutlet conduit 37 is constructed similarly to that described in acopending application of William A. Webb et al. entitled RemoteEquipment Maintenance, Serial No. 659,002, filed May 15, 1957, andassigned to the present assignee and now abandoned. The discharge sideof the pump 36 is coupled to the intake conduit 26 and manifold 22 ofthe reactor vessel.

In this example, the reactor vessel 20 is formed from a plurality ofspheroidal sections 38 which are welded together as shown to form thecompleted vessel. In order to minimize neutron-induced thermal stresseswithin the Walls of the vessels 20, which are of the order of six andone-half inches in thickness, a plurality of thermal shields, indicatedgenerally by the reference character 40, are disposed adjacent the innersurface of the reactor vessel walls. The thermal shields 40 conformgenerally to the inner configuration of the vessel walls and are spacedtherefrom and from one another in order to provide, in this example,flow channels therebetween for the passage of a peripheral portion ofthe circulating nuclear fuel. Inasmuch as the thermal shields 40 aresubjected to little or no pressure differentials, they are maderelatively thinner with respect to the vessel walls 20. A plurality ofbaflles 42 are disposed adjacent the lower or intake manifold 22 and aresuitably shaped for distributing the incoming slurry as indicated byflow arrows 44 throughout the interior areas of the vessel 20 and fordiverting a peripheral portion of this flow through the passages formedbetween the thermal shields 40 and adjacent the inner Wall of the vessel20. A neutron refiecting member (not shown) can be disposed adjacent thethermal shields to reflect peripheral neutrons back into the centralregion of the vessel 20 in order to im prove the neutron economy of thechain reaction.

The disposition of the thermal shields 40 in this manner substantiallyprevents impingement of fission-neutrons upon the adjacent vessel walls,Accordingly, the heating efiect of the impinging neutrons is developedalmost entirely within the thermal shields 40 which are not subject topressure stresses as are the walls of the pressurized vessel 20.Moreover, the heat developed within the thermal shields 40 is readilyremoved by the peripheral portion of the circulating fuel flowingthrough the channels therebetween. Alternatively, the thermal shields 40can be replaced by the shield arrangement disclosed and claimed in acopending application of W. P. Haass, entitled Reactional Vessel, SerialNo. 652,627, filed April 12, 1957, now Patent 3,075,909, dated January29, 1963, and assigned to the present assignee.

The pressurized reactional vessel 20 is mounted upon an annularsupporting collar indicated generally by the reference character 46 andmounted upon a biological shielding wall portion or support 48. Thismounting arrangement for the reactor vessel 20 and the physicaldistribution of the primary circulating loops and other equipmentassociated therewith are described in greater detail in a copendingapplication of W. A. Webb et al., entitled Reactor Plant, Serial No.659,004, and assigned to the assignee of the present application.

In order to drain the contents of the reactional vessel, a drain outlet50 disposed in the lower or intake manifold 22 is coupled to a series ofslurry drain tanks 52, through a conduit 54. When it is desired to fillthe reactor system, the slurry contained in the drain tanks 52 isreturned through another conduit 56 which is coupled to one or more ofthe circulating loop conduits 35. To aid in filling the reactionalvessel and associated loops,

an auxiliary slurry pump 58 is coupled into the conduit 56. The physicaldisposition of the drain tanks 52 relative to the nuclear power plantarrangement is described in greater detail in the last-mentionedcopending application. For the present, it may be pointed out that thedrain tanks 52 are provided in sufi'icient number to contain at leastall of the circulating nuclear fuel slurry of the system but are of suchsize that none of the tanks can contain a critical mass of slurry.Suitable neutronabsorbing material (not shown) is disposed betweenadjacent tanks in order to prevent the development of a chain reactionwithin the collective group of tanks when they are filled with thecirculating fuel.

In one exemplary arrangement, the fluid fuel contained within each ofthe drain tanks 52 is stirred constantly by individual agitators orstirrers 59 mounted adjacent the top of each of the tanks 52. The tanks52 and the agitators 59 desirably are hermetically sealed to preventleakage of biologically hazardous fluid and desirably are pro vided inthe form of that disclosed and claimed in a copending application of Meiand Widmer, entitled Sealed Agitator, Serial No. 672,661, filed July 18,1957, now Patent 2,907,556, and assigned to the present assignee.

The upper or outlet header 24 is fitted with an additional port 60 towhich a surge tank 62 is coupled by means of a conduit 66. In one formof homogeneous reactor system, the surge tank 62 comprises a relativelylarge volume which, however, is insufiicient to contain a critical massof the circulating fuel. When in operation, a vapor space 68 is formedin the surge tank, which conveniently contains a vapor of the vehicleemployed in suspending the aforementioned fissionable and fertileoxides. As a result, during a positive system transient within thehomogeneous reactor system, a surge of liquid into the tank 62compresses the vapor confined within the surge tank space 68, therebyrelieving at least partially the increased pressures developed withinthe system.

'A pressurizing vessel 64, which is coupled to the surge tank 62 by aconduit 67 connecting the vapor spaces thereof, is furnished with anumber of heating elements, indicated generally by the referencecharacters 70 and arranged for heating a portion of liquid, desirablythe same as the aforementioned liquid vehicle of the system. Thus, thereactor system is maintained at the desired operating pressure, byvaporization and expansion of the aforesaid vehicle portion. As acorollary function the pressurizing vessel 64 operates to maintain theaforementioned vapor'space or surge volume 68 within the surge tank 62.The pressurizing vessel 64 is provided With an inlet conduit 72 wherebythe vessel 64 is initially charged with the aforesaid vehicle portionand make-up vehicle is added to the pressurizing vessel as required.This make-up vehicle is necessitated by radiolytic decomposition of thevehicle within the system and the incomplete recombination of thecomponent gases of the vehicle.

Alternatively, the pressurizing vessel 64 and the surge tank 62 can bereplaced by the pressure regulating system claimed and disclosed in acopending application of Jules Wainrib, entitled Pressure ControllingSystem, Serial No. 677,942, filed August 13, 1957, now Patent 3,060,110,dated October 23, 1962, and assigned to the present assignee.

Referring now more particularly to FIGS. 2A, 2B and 2C of the drawings,the various auxiliary equipment associated with the aforedescribedhomogeneous reactor system, is illustrated schematically therein. In thearrangement of the homogeneous reactor system, illustrated in the latterfigures, the primary slurry pump 36 (FIG. 2B) is furnished with acapacity of approximately 8,000 gallons per minute which in conjunctionwith three other primary slurry pumps (not shown) disposed in a likenumber of similar circulating loop systems indicated generally by arrows25, produces a total rate of flow of approximately 32,000 gallons perminute. Inasmuch as the'reactor vessel 20 and the circulating loopstogether ti enclose a total volume of approximately 19,000 gallons, thecirculating fuel is recycled through the system in about one-halfminute.

In the normal operation of the invention, the circulating slurrycomprises a vehicle of deuterium oxide (D in which is suspended about300 grams of thorium oxide (T110 per kilogram of D 0 and approximatelyten grams of uranium oxide (U0 per kilogram of D 0, With the U0 beingadmitted uniformly with the T110 The uranium in this example is fullyenriched and contains upwards of 90% of U isotope. Added with theuranium oxide is a very small proportion of a palladium catalystemployed to promote internal recombination of the major proportion ofthe radiolytic vehicular gases, deuterium and oxygen. The uncombined orremaining radiolytic gases are employed to sweep fissional product gasesout of the system, as explainedhereinafter. The quantity of palladiumcatalyst, which is added in the form of the oxide (PdO) is of the orderof 0.001 gram per liter of slurry and can be replaced, if desired, byother suitable catalysts, for example, a platinum compound.

Accordingly, the system circulates a mixed oxide slurry with a totaloxide concentration in excess of 300 grams per kilogram of D 0 whichcorresponds to a solids content of about 3% by volume. The reactionalvessel and the circulating loops are maintained under a pressure in theneighborhood of 2,000 pounds per square inch absolute by operation ofthe pressurizing vessel 64. The pressurizing vessel 64, which desirablycontains only deuterium oxide, or other such vehicle employed in thehomogeneous system as noted heretofore, is separated from the liquid orslurry portion of the surge tank 62 by means of the steam space 68(FIG. 1) thereof, to which the conduit 67 is coupled, thus avoiding theoaking that would result if the circulating slurry itself were boiled inthe pressurizing vessel 64.

Leaving the reactional vessel. 20 the slurry stream branches into fourparallel identical circulating loops 25 only one of which is illustratedin detail. each loop can be isolated from the reactor by a pair of dualstop valves 328 (FIG. 2B) to permit certain types of remote orsemidirect maintenance, without shutting down the entire plant, to beperformed on one of the circulating loops, for example, in the mannerdescribed in the copending coassigned applications of McGrath et al.,Serial No. 659,003 entitled Semidirect Equipment Maintenance, filed May14, 1957, now Patent 3,090,740, dated May 21, 1963, and of Webb et 21;,Serial No. 659,002 entitled Remote Equipment Maintenance, filed May 14,1957.

Within the reactor vessel 20 part of the kinetic energy of the fissionalfragments is absorbed by the deuterium oxide molecules, some of whichare disassociated into deuterium and oxygen gases. For the most partthese radiolytic gases are recombined within the reactor system throughusage of the palladium catalyst noted above. However, the remainingportion of these radiolytic gases is removed together with certaingaseous fission products by means of the gas separators and conveyedthrough the conduit 31 to an external recombining unit associated with asuitable gas handling system (not shown). Suitable forms of gas handlingsystems adapted for recombining the radiolytic gases and for separatingand eliminating the fissioned product gases are disclosed and claimed inPatents 3,080,307 and 3,093,564 above referred to.

From the gas separators 30 the circulating slurry in each loop 25 isconducted to the respective steam generators 32 (FIG. 2C), as notedheretofore. The steam developed in the steam generators is conductedthrough the outlet conduits 37 and conduits 80 to suitable steamutilizing means, for example, one or more turbo-electric generators (notshown). A plurality of storage tanks 84, with two being shown forpurposes of illustration,

If desired,

are coupled to the steam generators by means of a conduit 86 forpurposes of draining the steam side of the steam generators in the eventof leakage of radioactive slurry into the steam side of the generatorsor other defeet. Any vapor contained at this time within the steamgenerators is removed through overhead conduit 88 to a steam generatordrain tank condenser 90. This vapor after being condensed in thecondenser 90 is conducted to the aforementioned drain tanks 84 through aconduit system 92. In the event that the contents of the steam generatordrain tanks 84 become radioactive the contents can be conveyed throughthe valved conduit system 94 to suitable means for concentrating orotherwise preparing the radioactive material for underground or oceanicburial.

As indicated heretofore, four circulating loops indicated generally bythe reference character 25, are associated with the reactor vessel 20;however, in the drawings only one of these loops are shown in detail,inasmuch as in this example these loops are substantially identical.However, as will be described subsequently in greater detail, theplumbing connections associated with one of the circulating loops 25differs slightly in that certain auxiliary equipment associated with thereactor system are coupled to only one of the circulating loops 25. Inthe case of the reactor system described herein, the most important ofthese auxiliary system is the fuel handling system which is illustratedin detail in FIGS. 2A, 2B and 2C of the drawings. In this arrangement,the operations performed within the fuel handling system are dividedinto four major categories of slurry storage, vehicular deuterium oxidestorage, concentration and dilution of the slurry, and transfer ofslurry or vehicle into and out of the primary system.

In this arrangement the fuel handling system comprises, inter alia,twenty drain tanks 52, although only five of these tanks are illustratedin FIGS. 2A and 2B. For purposes of exemplifying the invention the draintanks 52 are grouped into three functional categories in which a singledrain tank 52A (FIG. 2A) serves as a slurry accumulator tank. A total ofsix drain tanks SZ'B (FIG. 2B) serve as storage for concentrated slurryand the remaining thirteen tanks SZC serve as a repository for theslurry normally circulated through the reactor vessel 20 and the primarycoolant loops 25. The latter group of tanks SZC are capable ofcontaining the entire contents of the primary reactor system or about19,000 gallons and are normally empty during reactor operation in theevent the reactor system must be shut down under emergency conditions orfor purposes of maintenance or other contingency. The aforementioneddrain tank groupings together with the common header conduits 96, 110,144, and 182 are sometimes hereinafter referred to as the drain tankcomplex.

Each of these storage tanks 52' is provided with a stirring mechanism59, which has been described previously in connection with FIG. 1 of thedrawings. As indicated heretofore none of the generally vertical draintanks 52' contains sufficient volume to provide a critical mass whencompletely filled with the fissile material employed in the reactorsystem. Moreover, the twenty drain tanks 52. are arranged in a separatedor spaced array and desirably are provided with neutron absorbingmaterial therebetween in order to prevent a critical mass from beingformed collectively among the drain tanks 52. A suitable physicaldisposition of the drain tanks 52' is described in a copendingapplication of W. A. Webb et al., entitled Nuclear Reactor Plant, SerialNo. 659,004, filed May 14, 1957, now Patent 3,113,915, dated December10, 1963, and assigned to the present assignee.

In this example, the drain tanks 52 are each approximately three feet indiameter and about thirty feet in height, which dimensions positivelypreclude criticality in the slurry contained within each tank under anyconditions. Each tank is designed to withstand an operating pressure of1500 p.s.i. For purposes elaborated upon subsequently, the concentratedslurry contained within the drain tanks 52'B in this example isapproximately double the normal concentration of the slurry circulatedthrough the primary reactor system.

The drain tank 52'A serves as aforesaid, as a slurry accumulator tanknormally used to collect small volumes of slurry periodically dischargedor blown down from various components of the reactor system forcleansing purposes. All the tanks 52' are coupled to a drain header orconduit 96 through individual valved conduits 98. The aforedescribedgroupings 52A, 52'B and 52'C of the drain tanks 52' are preserved byinsertion of normally closed valves 100 (FIG. 2A), 108 (FIG.

2B) and l02 (1 16.213) within'the drain header 96;

The reactional vessel is coupled to the drain header 96 by means of aconduit 54 connected to the outlet port 50 of the lower reactionalvessel manifold 22 as described heretofore in connection with FIG. 1 ofthe drawings. To ensure quick and complete draining of each primary loop25, the suction side of each primary pump 36 and the inlet of each steamgenerator 32 are connected through a branched conduit system 104 to thedrain header 96. To control draining of the reactor system in thismanner a pair of stop valves 106 are inserted in each branch of theconduit 104 and in the conduit 54. In this manner the reactioned vessel20 and each of the four circulating loops are coupled through the drainheader 96, a normally opened valve 108 in the drain header 96, and thedrain tank inlet conduits 98C to the thirteen slurry repository tanks52C.

By suitably opening one or more of the valves 100, 102, or 108 in thedrain header conduit 96 the slurry accumulator tank 52'A in an emergencycan be utilized either with the group of six concentrated slurry tanksor with the group of thirteen slurry repository tanks 52C.

Thus, it will be seen that the drain header mainly serves to connect atotal of nine points of drainage, i.e., one at the reactor vessel 20itself and two at each of the primary loops 25, from the primary reactorsystem to the drain tanks 52'. Among the auxiliary functions of thedrain header 96 are transfer of deuterium oxide steam from a suitableevaporator (not shown) to the drain tanks 52' for heating andpressurizing the drain tanks 52' to prevent thermal shock upon contactby hot slurry drained from the primary reactor system, return ofcondensed deuterium oxide from a plurality of condensers 114 (FIG. 2A)and 115 (FIGS. 2B and 2C) presently to be described to the drain tanks52', and transfer of material from one drain tank to another in themanner presently to be desrcibed.

Each of the twenty drain tanks 52' also is coupled to a liquid header orconduit 110 by means of individual valved conduits 112. A small portionof slurry is drawn periodically from the drain tanks 52' and isconducted by means of the liquid header conduit 110 and a conduit 116 toa chemical processing plant 118 (FIG. 2C). At the processing plant 118this portion of slurry is chemically processed to remove fissionalproducts created during normal reactor operation. The reprocessed slurryof the chemical processing plant 118 can be conveyed to the primaryreactor system by means of a slurry pump 318 and valved conduits 120 and121. The conduits 120 and 121 conduct the reprocessed slurry to abattery of high head pumps, indicated generally by the referencecharacters 122, and thence to one of the circulating loops 25 by way ofa valved conduit 124 and one of the branched conduits 104 describedheretofore. When not being added to the primary reactor system in thisfashion, the output of the chemical processing plant 118 can be conveyedby means of the slurry pump 318 through a suitable cooler 126 (FIG. 2B),conduits 121 and 128, and a valved conduit 130 to the liquid header 110.Alternatively, as in this arrangement when the reactor is not inoperation, the output of the chemical processing plant can be conveyedthrough the cooler 126 and conduit 128 as before, and through anothervalved conduit 132 to the slurry accumulator tank 52'A.

However, when filling the primary reactor system including the vessel 20and associated circulating loops 25, slurry is withdrawn from the draintanks 52' in the reversed direction through the valved conduit 130 or132 or both, and through the cooler 126 and the associated conduit 128by means of the aforesaid battery of high head pumps 122 and theconduits 120 and 124. As pointed out previously, the conduit 124 iscoupled to one only of the circulating loops 25 through the associatedone of the branched conduits 104. The liquid header 110 is provided withvalves 134 and 136 (FIG. 2B),

which in conjunction with a valve 138 disposed in the conduit connection112A of the slurry accumulator tank, determine which of the three groupsof storage tanks SZ'A, 52B or 52C are coupled to the suction side M0 orthe high head pumps 122.

After the reactor system is filled the primary function of the liquidheader 110 is to provide for overflow from one storage tank 52' toanother in each group and to assure level equalization among the draintanks in each group. The liquid header also is employed to transferslurry vehicle, which in this case is deuterium oxide, to one or more ofthe drain tanks 52 for purposes of dilution or the like, to transferslurry as subsequently described from one tank to another, or to conveyslurry from one or more of the drain tanks to the chemical processingplant 118 in the manner described heretofore. The individual conduitconnections 112 which couple the storage tanks 52' to the liquid header110 extend to the bottom of each tank to permit almost complete liquidremoval therefrom. These extensions of the conduits 112 are indicated bydashed lines 142.

After the slurry has been subjected to chain reaction within thereactional vessel 20 and subsequently stored in the drain tanks 52, theslurry will release a considerable amount of heat due to radioactivedecay of the contained fission products. The decay heat is removed fromthe storage tanks 52' by condensation of that portion of the deuteriumoxide vehicle which is vaporized by the decay heat. In furtherance ofthis purpose, each of the drain tanks 52' is coupled to a vapor header144 through individual vapor conduit connections 146. That portion ofthe vapor header 144 which is coupled to the slurry repository tanks 52Cis isolated from the remainder of the vapor header 144 by a pair of stopvalves 148 (FIG. 23). By the same token that portion of the vapor headercoupled to the slurry accumulator tank 52A can be isolated from theremainder of the vapor header 144,. if desired, by means of a normallyopen valve 150 (FIG. 2A).

More specifically, any heat developed in the slurry accumulator tank 52Aand in the six concentrated slurry tanks 52B is removed by conveying theresultant deuterium oxide vapor from these tanks 52'A and 57/13 throughthe associated valved conduit connections 146, the vapor header 144, anda valved inlet conduit 152, to a slurry entrainment separator 154 (FIG.2A). The slurry entrainment separator 154 is a conventional,centrifugaltype device and therefore will not be described in detail.The entrained liquid output of the entrainment separator is returned tothe drain header 96 or to the liquid header 110 through a conduit 156and valved conduits 158 or 160, respectively. Thus it is seen that theentrained slurry can be returned to the drain header 96 or to the liquidheader 110 and thence to one or more of the storage tanks SZA or 52'B byopening an appropriate one of valves 162 or 164 of the aforesaidconduits 158 or 160.

The vapor from which any entrained slurry has been removed is thenconveyed to an associated drain tank condenser 114 through a conduit166. After being condensed the liquid deuterium oxide is conveyed fromthe condenser 114 through a valved outlet conduit 168 and 13 the valvedconduit 15% or 1450 described heretofore to either the drain header 96or the liquid header lift. A check valve 176 is disposed in the valvedconduit N8 in order to prevent reverse flow of slurry from theentrainment separator 154 which slurry would tend to clog the heatedportions of the condenser 114.

In a similar fashion the vapor removed from the slurry repository tanks52'C and the associated portion of the vapor header lad is conducted inparallel paths through valved conduits 171 to a pair of slurryentrainment separators 172 (FIGS. 23 and 2C). The efiiuent vapors of theslurry entrainment separators 1'72 are conducted to associated ones ofthe drain tank condensers 115. The outputs of the drain tank condensersH5 and of the slurry separators 172 are conveyed respectively throughconduits 1'74 and 176 and one of the valved conduits 1'78 and 189 toeither the drain header 96 or the liquid header 11h, as desired. Asstated heretofore the vapor generated in the repository tanks 52'C andconducted through the vapor header 144 is normally isolated from thebalance of the vapor header by means of the stop valve 148. Thus, itwill be seen that two drain tank condensers 115 and associatedcomponents are reserved for use with the repository tanks 52'C while onecondenser 114 and entrainment separator 154 are employed with thebalance of the drain tanks. This arrangement is necessary due to thelarger number of repository tanks SZC and to the fact that these tanksnormally are initially employed to store very hot slurry, which moreovercontains a quantity of fission products as a result of having beensubjected to the fissional process within the reactor vessel 26Alternatively, instead of employing the en trainrnent separators 154 and172 for respective ones of the drain tank condensers lid and 115, aflash section can be built into the vapor space in each drain tank and asuitable entrainment separator (not shown) can be included in this spacethereby eliminating the external separators 154 and 172.

The storage tanks 52' are provided in addition, with a relief header,denoted by the reference character 182. The relief header 132 isconnected to the individual storage tanks 52 by means of conduits 134containing relief valves 185 and coupled respectively to the vaporconduits 146 of each tank 52. The relief header 1&2 is coupled to aninput relief header 1186 associated with the deuterium oxide storagetank condenser 133 (FIG. 2C) presently to be described. Likewise,coupled to the input relief header 185 by means of suitable connections(not shown) are other components of the reactor system, as indicated byconduit segments 1%. In order to satisfy the ASME code there are novalves in the relief headers 182 and 136 other than the relief valves185.

Storage space for the slurry vehicle or in this case heavy water isprovided by a plurality of storage tanks 192 (FIG. 2C) with three beingemployed in this example. During normal operation of the reactor systemthese tanks are approximately half full, whereas during a plant shutdownthe tanks are completely full. A storage tank condenser 188 is coupledto a common storage tank outlet 1% through a conduit 1%. Any vaporformed in the storage tanks 192 is conducted to the overhead condenser18% by means of an overhead conduit system 198. The outlet conduit 194-is coupled to the suctional side of the deuterium oidde pump 2% througha valved conduit 202. When supplying deuterium oxide vehicle to thechemical processing plant 118 for preparation of slurry, makeuppurposes, or the like, the conduit 292 is coupled thereto throughanother valved conduit 2%. By means of the pump 2% the vehicle storedwithin the tanks 192 for purposes noted hereinafter, can be supplied tothe primary reactor system through a conduit 2% which is connected tothe associated branched conduit 104 of one of the circulating loops 25.A check valve 208 is coupled in the conduit 206 in order to preventreverse flow from the primary reactor system or from the high headslurry pumps 14 122. With this arrangement the primary reactor systemcan be filled initially with deuterium oxide vehicle during theprocesses of starting up and shutting down, presently to be described.

Due to their auxiliary function as catch tanks for relief purposes thedeuterium oxide storage tanks 1% must be limited to a maximum diameterof three feet in this example inasmuch as it is conceivable that thecirculated fuel slurry could be conducted to these tanks by way of theinput relief header 186 and storage tank condenser 188. As pointed outpreviously in connection with the drain tanks 52, a tank of thisdiameter is positively incapable of containing a critical mass of theaforesaid fuel slurry. The storage tanks 192 also are employed asaccumulators for liquid or vapor discharged from other components of thereactor plant either normally or during relief situations. Employment ofthese stoage tanks 1% as relief volume is desirable inasmuch as theiroperating pressure is relatively low and ranges from. about atmosphericto p.s.i.a. The condenser 188 thus serves to limit the maximum pressureduirng a relief operation and also to remove radioactive decay heat bycondensation of the attendant vapors, developed within any slurry thatmay be conducted to the deuterium oxide tanks.

The initial quantity of deuterium oxide or other slurry vehicle requiredfor the reactor system is added to the storage tanks 192 from a primarystorage system (not shown) by mean sof a conduit 216) which joins theoverhead conduit system 198 of the storage tanks 192. Another conduit212 is joined to the outlet side of the deuterium oxide pump 2%, wherebya quantity of the deuterium oxide is supplied to a suitabel gas handlingsystem, such as that noted previously, where it serves as a diluent forthe radioactive gaseous fission produce conveyed through thelatter-mentioned system. The latter portion of deuterium oxide can besupplied to an external evaporator (not shown) or one associated withthe gas handling system and the deuterium oxide steam not used in thegas handling system is returned through a conduit 214- to the storagetank condenser 188. The D 0 storage tanks and associated components inaddition are pressurized by the aforesaid evaporator to prevent vaporbinding in the D 0 pump 2%.

Slurry concentration and dilution is performed in the drain tank complexor alternatively in the slurry concentrator 220 (FIG. 2a), presently tobe described. The operations of concentration and dilution areundertaken in order to facilitate starting up and shutting down theprimary reactor system. Concentration in. the drain tank complex is doneby boiling and utilizes the radioactive decay heat of the slurry itself.The resultant vapor is condensed in the drain tank condensers 114 and115 and is collected in the deuterium oxide storage tanks 192. Infurtherance of the latter purpose valves 222 (FIG. 2a) and 224 (FIG.21)) disposed in the outlet conduits 168 and 174;, respectively, of thedrain tank condensers 114 and 115 are closed; and the condensate,leaving the drain tank condensers 114 or 115, is conducted instead tothe input relief header 186 and through the deuterium oxide storage tankcondenser 188 through a valved conduit 23% and through conduit 226 (FIG.2a) or 228 (FIG. 25), respectively. With this arrangement in utilizingthe decay heat as aforesaid the desired slurry concentration can beobtained in each group of tanks 52'B and 52C.

As indicated heretofore, the slurry contained Within the smaller groupof drain tanks 52B is about double that initially conveyed from theprimary reactor system to the other group of tanks 52'C. When theconcentration of slurry contained within the repository tanks 52C isincreased in the aforesaid manner to that desired for the concentratedslurry tanks 52'B, for example, the vapor pressure developed in thetanks 52C can be employed to transfer the contents of the latter tanksto the other group or groups of tanks 5233 or 52'A depending upon thestorage voume required. In furtherance of this purpose,

valves 334 and 336 (FIG. 2b) disposed in the drain and vapor conduits98c and 1460 of each repository tank 52'C are closed, whereupon thevapor resulting from decay heat of the slurry within these tanks tendsto accumulate at the top thereof. At the same time a valve 341 in theextended connecting conduit 1120-142 of each tank 52'C and the valve 136of the liquid header 116 are opened whereupon the increasing vaporpressure forces slurry from the tanks 52'C into the liquid header 110.At this point the other liquid header valve 134 can be opened and byopening selected ones of valves 138 (FIG. 2a) and 348 (FIG. 2b) disposedin the connecting conduits 112a and 11% of the accumulator tank 52'A andof the concentrated slurry tanks 52'B, the slurry forced out of tanks52'C can be deposited in one or moer of the tanks 52'A and 52B.Alternatively, valves 134 and 342 (FIG. 2b) can be closed, and uponopening valves 320 (FIG. 2b) and 344 (FIG. 2a) in the conduits 130 and132, respectively, the contents of the repository drain tanks 52'C, ifrelatively small in quantity, can be conveyed directly to the slurryaccumulator tank 52'A in bypassing relationship with the concentratedslurry tanks 52B. The accumulator tank 52'A is then isolated, as innormal operating conditions, from the concentrated slurry tanks 52B byclosing the valve 138 situated in the associated connecting conduit112a. Alternatively, slurry can be transferred among the individualdrain tanks 52 of the drain tank complex by coupling selected ones ofthe tanks, from which material is, to be removed, to the liquid header110 by opening associated ones of valves 138, 340 and 341 in theconnecting conduits 112a, 11212, and 112C, respectively. The slurrypumps 122 are then energized to draw material from the selected tank ortanks via the liquid header 110 and conduits 130 and 128 to the drainheader 96, after opening valves 320 and 342 (FIG. 2b). The material thenis deposited in selected ones of the tanks 52' through conduit 332 (FIG.20) and one or more of the conduits 98a, 98b, and 98c upon openingappropriate ones of their individual valves 333, 335, 337 and 334respectively, and upon opening appropriate ones of the valves 100, 102and 108 in the drain header 96. Still another method of transferringmaterial among the tanks or removing material from the drain tankcomplex involves increasing the pressure of pressurizing D steamsuppliedv to the drain header 96 through conduits 317 (FIG. 2b) from anexternal evaporator (not shown) and openingappropriate ones of theaforesaid valves.

Dilution of the slurry in each group of tanks in the drain tank complexis accomplished by bypassing the pump 200 and withdrawing liquiddeuterium oxide from the storage tanks 192 by gravity through conduits262 and 306 to the liquid header 110. From this point the diluentdeuterium oxide can be diverted to selected groups or individual ones ofthe drain tanks 52 by suitable manipulation of valves 134, 136 and 138.Alternatively, liquid D 0 can be removed from the tanks 192 byincreasing the pressure of D 0 steam supplied thereto through conduit.In the event that decay heat is not available for concentrating theslurry in this fashion, a slurry concentrator 220 iscoupled to the lowermanifold 22 (FIG. 1) of the reactional vessel through a valved conduit238 and a second valved conduit 240. Thus during normal reactoroperation a quantity of slurry is supplied through the concentrator 220upon opening valves 242 (FIG. 2b) and 244 (FIG. 2a) disposedrespectively in the conduits 238 and 240. When, however, the primaryreactor system is not in operation and when sufiicient decay heat isunavailable for concentrating the slurry as aforesaid, the slurry isdrawn from the drain tanks 52' through the connecting conduits 112, theliquid header 110, and associated conduits 130 and 128 by means of thehigh head pumps 122. A valve 246.(FIG. 2b) in the outlet conduit 124 ofthe pumps 122 is closed and thus causes the slurry to flow through avalved connecting conduit 248 coupled be- 16 tween the outlet conduit124 and the inlet conduit 240 of the slurry concentrator 220. Closingthe valve 246 of course, causes the slurry to flow to the concentrator220 in bypassing relation to the primary reactor system.

The slurry concentrator 220 in this arrangement comprises a battery ofhydroclones (not shown), or the like, which are designed for highpressure operation. The hydroclone arrangement, or other centrifugaldevice, separates the slurry into dilute and concentrated fractions,which exit from the concentrator 220 through the conduits 256 and 252(FIG. 2A), respectively. The dilute and concentrated slurry streams areconducted through suitable coolers 254 and 256 to prevent flashing ofthe slurry streams in the depressurizing or letdown devices, presentlyto be described. The coolers 254 and 256 comprise, for example, ordinaryheat exchangers which are designed for the operating pressures of thereactor system and are cooled by cooling water supplied thereto by meansof conduits 258 and 260.

From the cooler 256 the concentrated slurry stream, which is aboutdouble the normal slurry concentration of the primary system, isconveyed through a slurry letdown device indicated generally by thereference character 262. The letdown device 262 consists of relativelysmall diameter tubing inserted in a suitable cooling medium (not shown)to prevent flashing of the concentrated slurry and having sufiicientlength to induce the desired pressure drop. At this point theconcentrated slurry is added to the drain header 96 and thence to thedrain tanks 52 through a valved conduit 264, after the pressure of theslurry has been reduced by the letdown device 262 from the reactoroperating pressure of 2000 psi. to 600 psi. or the operating pressure ofthe drain tanks 52'. A check valve 266 is coupled in the drain header 96between the conduit 264 and the remainder of the components connected tothe drain header in order to prevent reverse flow from the drain headerto the letdown device and associated components.

A small portion of the slurry concentrated in this fashion isperiodically withdrawn from the outlet stream of the letdown device 262through a valved conduit 267. The latter conduit 267 couples theconcentrated letdown device to a slurry measuring tank 268 (FIG. 2A).With the measuring tank 268 the amount of concentrated slurry whichperiodically is extracted from the primary reactor system and isconducted to the chemical processing plant 118 through valved conduit270 can be proportioned precisely. Any part of the contents of theslurry measuring tank 268 which is not conducted to the chemicalprocessing plant is conveyed to the drain header 96 by means of a valvedconduit 272.

The dilute slurry stream issuing from the concentrator 229 is conductedafter passing through the cooler 254 to a depressurizing or letdowndevice indicated generally by the reference character 274. The letdowndevice 274 is similar in structural detail to the concentrated slurryletdown device 262 and accordingly a further description is dispensedwith. A relatively small portion of the output of the letdown device 274in this arrangement is conveyed to an evaporator 276 (FIG. 2A) through avalved conduit 278. The remainder of the diluted output is conveyed tothe D 0 storage tank 192 through a valved conduit 322.. In theevaporator 276, a portion of the slurry vehicle or deuterium oxide isvaporized by means of process steam (H O) supplied to heating coil 280.The remaining slurry, after a portion of the vehicle is removed, isconveyed from the bottom or outlet of the evaporator 276 through avalved conduit 282 to the slurry accumulator tank 52'A. The vaporgenerated within the evaporator 275 is conveyed to a purge condenser284. In the purge condenser 284, the vaporous vehicle is condensed toform liquid deuterium oxide which is used for purging various componentsof the reactor system in order to prevent accumulation or settling outof slurry particles therein. The purging liquid is supplied to theaforesaid components, as indicated partially by purge header 316 andterminal conduits 386, by means of a high head purging pump 283. Thepurging liquid first is conducted through a cooler 290 in order toprevent vapor binding in the high head pump 288. A check valve 292 isdisposed in a conduit 2% between the cooler 290 and the pump 2% in orderto prevent reverse flow in the event of pump failure. The purging systemjust described in this example serves desirably as an auxiliary purgingsystem for use during reactor shutdown or other contingency. Duringnormal reactor operation, purging liquid is supplied from a fissionalgas handling system usually associated with the reactor plant, such asone of the gas handling systems disclosed and claimed in theaforementioned copending applications of D. F. Rinald and of J. Weismanet al.

In the event that it is not desired to separate the slurry into lightand heavy fractions by means of the concentrator 220, the concentratorcan be bypassed by means of a valved conduit 2%; and slurry can besupplied directly from the primary reactor system to the cooler 25s andconcentrated slurry letdown device 262 for depressurizing in the mannerdescribed heretofore and conveyance for instance to the chemicalprocessing plant 118 via the slurry measuring tank 268. The heavy ordilute fractions issuing from the concentrator 220 can be returned tothe primary reactor system, for purposes of starting up or shutting downthe reactor as explained more fully here inafter, by means of valvedconduits 2% and 300, with the former conduit being coupled to thesuctional side of one of the primary loop pumps 36. The valved conduits298 and 300 are coupled respectively to the outlet conduits 250 and 252of the slurry concentrator 220, and a check valve 302 or 304 isconnected in each of the conduits 2.93 and 300 in order to preventreverse iiow from the primary reactor system to the slurry concentrator22% or to the slurry coolers 254 and 25%.

The normal process of transferring material into or out of the primaryreactor system is accomplished by means of the D storage tank pump 200and the high head slurry pumps 122 (FIG. 2C), in conjunction with theslurry concentrator 220. The D 0 storage tank pump 200 is provided inorder initially to fill the primary reactor system during start-up withheavy water at a relatively high rate and low pressure oi about 150p.s.i.a., while the high head slurry pumps 122 are employed to transferconcentrated slurry from the drain tanks 52'? to the primary reactorsystem at a considerably lower rate. The concentrated slurry is added atabout 2000 p.s.i.a., to which pressure, the initial D 0 fill is raisedby operating the pressurizing vessel 64. At the same time, the slurrywithdrawn from the primary reactor system through the conduit 238 is fedthrough the slurry concentrator 220, and a selected one of the light andheavy fractions of the slurry concentrator output is returnedrespectively through the conduits 300 and 29% from the concentratorconduit 250 or through the conduit 2% from the other concentrator outletconduit 252, as the case may be, to the primary reactor system.

As will be described presently, in greater detail, the aforementionedlight fraction is fed back into the primary reactor system when it isdesired to dilute gradually the circulated fuel slurry in order to shutdown the reactor in the normal manner. At this time the high head pumps122 are employed to pump D 0 or very dilute slurry from the storagetanks 192 to the primary system through conduits 202, 306, 130, 12-8,120, 12 i, 248 and 238. On the other hand, the aforesaid heavy fractionis fed back to the primary system in order to increase gradually theconcentration of the circulated slurry when starting up the reactor.However, during an emergency the entire contents of the primary reactorsystem may be transferred to the drain tanks 52' rapidly, in thisexample in about fifteen minutes, simply by opening the nine pairs ofdrain valves 106. This emergency drainage procedure is avoided if at allpossible, because of the high thermal stresses and the erosional damageinduced in the drain tank complex and associated components of thesystem, when contacted suddenly by the large volume of heated slurryfrom the primary reactor system. During normal reactor operation thisproblem is not encountered because only relatively small volumes ofslurry are removed from the primary reactor system by means of theslurry concentrator 220 and the dilute and concentrated slurry letdowndevices 274 and 262, respectively.

in the course of normal reactor operation, approximately 47 gallons ofthe circulated slurry are removed daily, in this example, and conveyedto the chemical processing plant 118 for removal of fissional productsand other accumulated impurities. This quantity of slurry is selectedfor processing on the basis that the entire contents of the primaryreactor system, or about 19,000 gallons will be reprocessed over a400-day cycle. As indicated previously, [the quantity of slurry "thusextracted each day is determined through use of the measuring tank 268.The primary reactor system is replenished with an equal volume of freshslurry from either the concentrated drain tanks SZB or from the slurrypreparation facilities (not shown) of the chemical processing plant 118.

When starting up the aforedescribed reactor system, the primary systemis initially filled by means of a so called push-pull method. In brief,this procedure consists of filling the reactor vessel 20 and. the fourasso ciated primary circulating loops 25 with a slurry vehicle such asliquid deuterium oxide, supplying an overpressure by means of thepressurizer 64 described herefroiore in connection with FIG. 1 of thedrawings so that the primary pumps 36 can be started, and finiallyadding a slurry of double the final concentration from the drain tanks52'B while at the same time extracting dilute slurry by means of theconcentrator 220 until the desired openalting concentration of thereactor is attained. In the following 'Ilables I and H the deuteriumoxide requirements and the capacity of the several systems andsubsystems illustrated in FIG. 2 of the drawings are given:

TABLE I D 0 Requirements Volume at Weight, 60 F., pounds gallons IrimaryReactor System 19,100 176, 700 D20 Slurry Preparation 7,150 66,100Minimum for Auxiliary Equipment (approx)- 1, 500 13, 900 Excess forAuxiliary Equipment and Reserve Slurry 900 8, 300

Total 28, 050 205, 000

TABLE II Various System and Sub-System Capacities Gal. Primary reactorsystem 19,100 D 0 storage tanks (192) 21,400 Drain or repository side ofdrain tank complex (SZC) 19,100 Concentrated slurry side of drain tankcomplex (SZA and 5213) 10,300 Auxiliary equipment 2,000

From the preceding tables, it will be seen that the initial deuteriumoxide requirements total 28,650 gallons, in this exemplary arrangement.The heavy water is fed into the D 0 storage tanks 192 from which 5,650gallons are Withdrawn to four of the slurry repository tanks SZ'C bygravity flow through the outlet conduits 15 4 and 202 of the D 0 storagetanks 192 and a valved conduit 306 coupling the last-mentioned conduit202 to the liquid header 110 of the drain tank complex. A check valve308 (FIG. 2C) is disposed in the connecting conduit 306 to preventreverse flow through the conduit 306 from the liquid header 110 and thednain tank complex. The aforementioned 5,650 gallons are withdrawn, ofcourse, as the D storage tanks 192 are being filled, inasmuch as thetotal volume of the storage tanks 192 in this example, is less than theinitial D 0 requirements. An additional 1,600 gallons are sent by meansof the D 0 storage pump 200 and conduit 212 to the evaporater (notshown) employed for example in one of the aforementioned gas handlingsystems. The latter quantity of D 0 initially supplies openating purgingliquid until the gas handling system attains normal operatingconditions.

Prior to adding slurry to the reactor system, it is necessary to obtaina supply of purging water, as aforesaid. As an alternative supply ofpurging water, a portion of the heavy water contained within the storagetanks 192 is transferred by gravity to the evaporator 276 describedheretofore in connection with FIG. 2A, by means of a valved conduit 310(FIGS. 2A and 2C). In the evaporat or 276 the heavy water is vaporizedand subsequently condensed in the purge condenser 284 as describedheretofore in connection with the dilute slurry fraction vehicle. The D0 withdrawn from the storage tanks 192, is conducted first to theevaporator 276 in order to remove from the purge water any slurryparticles which may have accumulated in the storage tanks. From thecondenser- 234 the heavy water is conducted through the cooler 290 andsupplied to the suction side of the high head pump 288 for purgingpurposes. From the pump 288 the condensed purge liquid is carried to apurge storage tank 312 through an outlet conduit 314. In the purgestorage tank 312 a. pressure in excess of the reactor operating pressureof 2,000 p.s.i.a. is maintained by means of a helium or another inertgaseous atmosphere supplied from a suitable pressurized source (notshown). At this point, high pressure water is now available to the purgeheader 316 which is coupled to those components {of the reactor systemwhich require purging, for example, the pumps and the entrainmentseparators as indicated by the partial conduits 286.

After a supply of purging water is made available, approximately 7,250gallons of heavy Water are transferred to the chemical processing plant113 for preparing the concentrated slurry which is employed as explainedhereinafter for filling the primary reactor system. In order to obtainthe last-mentioned quantity of heavy water, the 5,650 gallons alreadypresent in four of the drain tanks 52C plus the steam condensed duringpressurization of the D 0 storage tanks and the drain tanks aresupplemented by heavy water from the D 0 storage tanks 192. Thepressurizing steam is supplied to the drain tanks 52' from the drainheader 96 via conduit 317, and to the storage tanks 192 through theconduit 214 from a suitable evaporator (not shown). However, at least19,100 gallons must be left in the storage tanks 192 for initiallyfilling the primary reactor system with the slurry vehicle. The slurryis then returned from the processing plant 118 by meansof the transferpump 318 (FIG. 2C) coupled the outlet conduit 121 of the slurryprocessing palnt. From the pump 318, the concentrated slurry isconducted to the liquid header 110 through the conduits 128 and 130after opening valves 320 and 342. The valve 136 of the liquid header isthen closed ahdthe valve 134 thereof opened with the result that theconcentrated slurry is conducted into the concentrated slurry tanks 52Bthrough the individual conduits 112B. At this time the slurryaccumulator tank 52A is isolated from the liquid header 110 by closingits associated valve 138.

The primary reactor system including the reactor and associatedcirculating loops is now filled with the 19,100 gallons of deuteriumoxide or other slurry vehicle contained in the D 0 storage tanks 192, bymeans 20 of the D 0 storage pump 200. Subsequently, the pressurizingvessel 64 is charged with deuterium oxide from a high pressure D 0storage tank (not shown) via conduit 321. The heaters 70 of thepressurizing vessel 64 are then energized to induce an initial pressureof several hundred pounds per square inch within the primary reactorsystem or a pressure sufficient to prevent vapor binding in thecirculating pumps 36. The full system pressure of 2,000 p.s.i.a. shouldnot be applied to the primary system until a temperature of at least 200F. is attained in order to avoid the tendency of the structural steelsto fracture due to brittleness at relatively low temperatures.

- For the same reason, no slurry or circulating fuel is permitted in theprimary reactor system until the aforesaid 200 F. temperature level isreached. This precludes the possibility of a sudden pressure surge orpositive system transient before the minimum temperature mentioned aboveis attained.

Accordingly, it is necessary to supply external heat to the primaryreactor system before adding any slurry thereto inasmuch as the initialreactor filling of deuterium oxide is added substantially at roomtemperature. In order to avoid excessive thermal stresses in the heavywalls of the reactional vessel 20, a heating rate of 50 F. per hour isemployed. One arrangement for supplying the necessary heat consists offilling the reactor system with relatively cold deuterium oxide or otherslurry vehicle, operating the primary pumps 36, and thus adding heat aspump Work. Alternatively, or in conjunction therewith, heat can be addedto the primary system through the steam generators 32 by means ofprocess (H O) steam supplied to the steam side of one or more of thesteam generators from an external boiler arrangement (not shown).

Before slurry can be introduced into the primary reactor system from theconcentrated slurry drain tanks 52B the repository drain tanks 52C mustbe prepared as a safety precaution, for the possibility of an emergencydrain. In furtherance of this purpose the tanks of 52C are maintained at250 F. or greater at all times. This is accomplished by emptying thedrain tanks 52C and continuously heating them with p.s.i.a. D 0 steamsupplied to these tanks through the conduit 317 from an externalevaporator, as aforesaid. To heat the tanks 52C from room temperature or60 F. to 250 P. requires the heat equivalent of approximately 15,000pounds of D 0 vapor and the capacity of the external evaporator is suchthat this amount can be generated in fifteen minutes. However, whenthermal equilibrium is once obtained, D 0 vapor is condensed at the rateof only pounds per hour in maintaining a temperature of 250 F. in theentire group of the thirteen repository tanks 52C. Additionally, thetanks 52C should be isolated from other portions of the reactor systemso that these other portions will not be subjected to thermal stressesinduced by drainage of hot slurry from the primary reactor system intothe repository tanks 52C. Moreover, the drain tanks 52B should likewisebe isolated at this time to avoid diluting the concentrated slurrycontained therein.

Slurry is now transferred from the concentrated slurry tanks 52B to theprimary reactor system by means of the high head pumps 122. As theconcentrated slurry is added to the primary reactor system, anequivalent amount of the initial deuterium oxide filling is removed fromthe primary system and this is continued until the desired slurryconcentration is obtained. Removal of the initial deuterium oxidefilling is accomplished by means of the slurry concentrator 220 which isdesigned to withstand an operating pressure in the neighborhood of 2,500p.s.i.a. More specifically, as the concentrated slurry is added to theprimary reactor system from the drain tanks 52B through conduits 11213,the liquid header 110, the high head pumps 122, and the conduit 124 asimilar quantity of liquid is extracted from the reactor system byopening valves 242. and 244 in the conduits 233 and 240, respectively,whereupon a quantity of the initial filling is conducted to the siurryconcentrator 22%. The heavy or concentrated fraction of the slurryconcentrator output is returned to the suctional side of one of theprimary pumps 36 through the conduits 252 and 29?; upon openingappropriate valves therein. The light or dilute output of the slurryconcentrator 22% at this time is conducted to the cooler 254 and thedilute slurry letdown device 274, and thence is returned to the Dstorage tanks 192 through the storage tank conduit 198 and a valvedconduit 322 (FIGS. 2A and 2C). It will be seen from this arrangementthat no additional pump is required for conveying fluid from the primarysystem to the slurry concentrator 22G) inasmuch as one of the primarypumps 36 supplies the driving head.

For a normal shutdown operation the flow paths of the light and heavyfractions issuing from the slurry concentrator 220 are reversed suchthat the heavy fraction is conveyed to the drain tanks 52 by means ofthe cooler 256, the concentrated slurry letdown device 252, and thedrain header 96. On the other hand, the light fraction of the slurryconcentrator 22 is conveyed through conduits 250, 390 and 298 to thesuctional side of one of the primary pumps 36. In this manner, theconcentration of the slurry circulating through the primary reactorsystem is diluted gradually until the reactor becomes subcritical. Ofcourse as slurry is conveyed to the slurry concentrator 220, during theshutting down procedure, an equivalent amount of deuterium oxide orother vehicle is added as a diluent to the primary system from the D 0storage tanks 192 by operating the high head pumps 122, as describedpreviously.

During start-up of the reactor, slurry is fed to the primary reactorsystem at a concentration of approximately 600 grams of thorium oxideand uranium oxide per kilogram of D 0 and at a rate of 30 gallons perminute or less until criticality is reached. During the time in whichconcentrated slurry is added in this fashion the liquid level in thepressurizing vessel as is maintained constant by controlling the rate ofdiluted slurry passing through the letdown device 274. The averagetemperature in the primary system during this portion of the fillingoperation has risen to approximately 300 F. so that criticality isobtained in the reactor vessel 20 at an average slurry concentration ofabout 40 grams per kilogram of D 0 as shown at point 327 on curve 338 inFIG. 3 of the drawings. At this point, of course, the eifectivecoefficient of criticality (k g) is equal to unity. The slurry mustthereafter he introduced at a reduced rate not only to avoid largeinputs of reactivity to the reactor system but also to limit the rate oftemperature rise to 50 F. per hour and to maintain (keff 1)- Theaddition of concentrated slurry is continued until the operatingtemperature of 522 F., at substantially zero power level is reached.This point is designated as the low critical concentration and occurs inthis arrangement at a slurry concentration of about 70 grams perkilogram of D 0 as shown by point 319 on curve 329.

At the aforedescribed low critical concentration, in accordance with theinvention, the reactor is operated at a substantial percentage of ratedpower output for a period in the neighborhood of two hours or untilsufiicient quantity of the fissional product, iodine 135, is generatedfor purposes hereinafter to be explained in detail. The iodine, isotope,together with a relatively smaller quantity of xenon 135, is animmediate product of the fissioning atoms of one or more of theaforementioned fissionable isotopes, such as uranium 235 which is mixedinitially with the fuel slurry. When the low critical concentration hasbeen reached as aforesaid and after the aforementioned period ofoperation, the reactor temperature is permitted to rise until thedecreased moderating ability of the deuterium oxide vehicle causes thereactor to become subcritical. This is due to the phenomenon associatedwith the well-known negative temperature coefficient of reactivity,described heretofore and utilized under normal conditions to control thereactor system described herein. The temperature of the primary reactorsystem is permitted to rise by removing only enough heat from theprimary circulating loops 25 to prevent excessive pressure in theprimary systems. This is accomplished in this example by withdrawingonly enough steam from the steam generators 32-, by suitable setting ofvalves 324 in their outlet conduits 3'7, to limit the pressure andtemperature of the steam generators to the design limitations of 900p.s.i.a. and 532 F. At the same time, valves 326 in the gaseous outputconduits 31 of the gas separators 30 are closed to prevent the removalof gaseous fissional products, particularly xenon 135, from the primaryreactor system. By removing only a small proportion of power in the formof heat from the primary system, a resultant temperature rise isencountered which causes the reactor system eventually to shut down. Theresultant expansion of the D 0 vehide-moderator is illustrated in thefollowing Table III, which expansion results in a negative temperaturecoefiicient of reactivity as explained previously.

TABLE III D 0 Volume Expansion Due to Temperature Increase Percent 60 F.to F 1.8 60 F. to 300 F 9.3 60 F. to 522 F. 33.8 120 F. to 522 F 31.4300 F. to 522 F 22.8

When operated in accordance with the invention the average reactortemperature will rise slightly above the operating temperature of 522 F.at zero power level. At this time the reactor becomes subcritical as aresult of the decrease in D 0 moderating ability, and a limited amountof subsequent fissions plus the decay heat evolved by fissional productsgenerated during the period of criticality will maintain the temperatureof the reactor system slightly above the operating temperature tomaintain the reactor in a state of subcriticality. As soon as thereactor becomes subcritical, heat removal via the steam generators 32 isterminated substantially to aid in preventing the reactor from againreaching criticality at the low critical concentration of nuclear fuel.As the average reactor temperature falls, the build-up of reactor poisonin the form of xenon 135, in the manner explained below, operates toprevent the reactor system from returning to the critical conditionbefore the concentration of the fuel is increased.

After the low critical concentration point has been reached and thetemperature has been increased to the operating temperature asaforesaid, no further addition of concentrated slurry is made to thissystem until a substantial portion of the aforementioned iodine fissionproduct has decayed to xenon 135. Since both of these radioactiveisotopes are comparatively shortlived, having half-lives of 6.7 hoursand 9.2 hours respectively, there is a definite time after reactorshutdown when the concentration of the indirect fission product xenon135 reaches a maximum. As shown in FIG. 4 of the drawings in which theproduction of xenon 135 is plotted against time after reactor shutdown,the concentration of xenon 135 reaches a maximum in approximately 10hours after the reactor becomes subcritical at its low concentrationcriticality point as aforesaid. However, within two to three hours afterthe reactor is suocriticalized in the manner explained above the xenon135 poison fraction attains about one-half of its maximum concentration.This concentration is sufficient to maintain the reactor in asubcritical condition although the production of decay heat, of course,gradually is den: na c 135 B ns In the following Table IV, thehalf-lives and thermal neutronic capture cross-sections of the attendantisotopes are indicated:

TABLE IV Half-life Crosssection in Barns Iodine 6.7h Negligible Xenon9.2 h. 3.2 X Ces1um 2.0 X 10 y. Barium (Stable) 5 From the foregoingtable, it will be seen that the indirectly produced xenon isotope canserve as a reactor poison due to its extremely high neutronic capturecrosssection whereas the directly produced isotope iodine 135 has anegligible effect upon the neutronic economy of the operating reactorsystem.

In accordance with the invention use is made of the high neutroniccapture cross-section of the internally produced xenon 135 in order toprevent the reactor systern from becoming critical at a giventemperature, that is to Say, to reduce the temperature at which thereactor can criticalize, as the concentration of the slurry is increasedfrom the low criticality concentration point to the high concentrationpoint. As discussed heretofore, this is the desired operating pointbecause of the higher conversion ratio.

As indicated heretofore, in order to obtain sufficient xenon 135, it isnecessary to permit the reactor to remain subcritical for only about twoto three hours or until a sufficient quantity of xenon 135 is producedin accordance with the aforenoted radioactive decay equation. Otherwise,the xenon 135 poison will be destroyed almost as quickly as it isformed, by neutronic absorption, and an appreciable quantity of xenon135 will ordinarily not be accumulated for a considerable length oftime. By closing the gas outlets of the gas separators .39, the indirectgaseous fission products, particularly the xenon 135 isotope, are notremoved from the primary reactor system. This xenon 135, which isproduced as a result of operating the reactor for a time at the lowcritical concentration followed by maintaining the reactor in asubcritical or low power condition for an additional period, is thenemployed as a reactor poison until the concentration of the slurry isincreased to the high critical concentration point which, as illustratedby point 331 on the temperature curve 329 (FIG. 3) is approximately 300grams of thorium oxide per kilogram of deuterium oxide vehicle. Theslurry concentration then is maintained constant until a sufiicientportion of xenon 135, in one arrangement, decays to cesium 135 to permitthe reactor again to obtain criticality but this time at the highcritical concentration point. As shown in Table IV and in FIG. 4 of thedrawings, the xenon 135 having a half-life of only 9.2 hours, decaysrapidly so that the reactor can be returned to criticality, at the highconcentration criticality point, within approximately hours after thereactor has been subcriticalized at the low critical slurryconcentration. Alternatively the reactor is much more rapidlycriticalized at the high critical concentration, by permitting the gasseparators 355 to re move the xenon poisoning, after opening valves 326in 24 their outlet conduits 31, or by lowering the temperature in theprimary reactor system, or both.

After the average reactor temperature has increased to 522 R, which isthe operating temperature in this case, the temperature is thenmaintained constant by extracting power in the form of heat from theprimary reactor system by means of the steam generators 32. Withdrawingpower, or" course, increases the temperature drop across the reactionalvessel 26, as illustrated in FIG. 5 of the drawings, while the averagetemperature as shown by line 346 remains constant. However, the slurrytemperature at the vessel outlet manifold 24 (FIG. 1) rises linearly,line .348, with power level until approximately 580 F. is attained atfull power operation, and the slurry is supplied to the steam generators32 at sub stantially the outlet manifold temperature. On the other hand,the slurry temperature at the inlet manifold 22 (FIG. 1) decreaseslinearly with increased reactor power output, line 35%, to about 465 F.at full reactor power. The temperature of steam issuing from the steamgenerators 3?; decreases linearly with the quantity of steam utilized asshown by line 352; thus, the temperature of the steam likewise decreaseslinearly with increased reactor power, to about 445 F. at rated poweroutput.

If the primary reactor system were not poisoned in the aforementionedmanner, the average temperature of the slurry in the primary reactorsystem would rise to approximately 595 F. as indicated by peak 330 oftemperature curve 36!) illustrated in FIG. 3 0f the drawings. It willalso be seen from FIG. 3 that this maximum temperature is obtained asthe concentration is increased from the low critical concentration of 70grams per kilogram of deuterium oxide to only about 170 grams perkilogram of deuterium oxide. Thus, it would be seen that theconcentration would otherwise have to be increased very slowly andcarefully to prevent development of thermal strains in the reactorsystem, especially the vessel 20, and that the pressure of the reactorsystem would have to be increased greatly in order to prevent boiling ofthe deuterium oxide in the vicinity of the peak average temperature of595 F. (point 330) and even higher to prevent destructive cavitation inthe primary pumps 36. The other alternative would be to add a poison,such as a boronic or cadmium compound, from an external source whichwould require additional and complicated auxiliary equipment foraddition and subsequent removal of the poison from the primary reactorsystem. The addition of xenon from an external source, of course, israther difiicult due to its relatively short half-life and attendantshielding and storage problems.

The process of shutting down the reactor system under normal conditions,for example for purposes of inspection or maintenance of equipment,requires in the order of approximately ten hours. The normal reactordraining procedure comprises essentially the reverse of the stepsemployed in the starting-up procedure. Initially, the heat withdrawnfrom the reactor is substantially reduced so that the resulting increasein temperature will render the reactor subcritical. The reactor, then,is maintained in this condition by heat of radioactive decay for one totwo hours to permit production of xenon 135. The xenon 135 concentrationwill then build up sufliciently to add poison to the reactor and reduce,in the manner describedheretofore in connection with starting up thereactor, the maximum temperature in the primary reactor system as theslurry concentration is decreased from the high critical concentrationto a low concentration. The shonter waiting period can be used becausethe reactor usually has been opera-ted for an extended period at thehigh critical concentration and because the slope of the temperaturecurves is relatively less between the high critical and the peaktemperature points. The slurry concentration is reduced by pumping in D0 vehicle at the rate of 30 gallons per minute and by utilizing theslurry concentrator 220 to return the light or 25 dilute slurry fractionto the primary reactor system through conduits 3th and 2% and to conductthe heavy or concentrated fraction through the slurry letdown device 262for storage in the drain tank complex. In furtherance of this purpose,dilute slurry or slurry vehicle, as the case may be, from the D storagetanks 192 is fed into the primary reactor system by means of the highhead pumps 122 and associated conduits. The concentration of slurrywithin the primary system should be reduced as far as practical in orderthat flushing of the primary system is not required subsequent to thenormal draining procedure. Obviously, the dilution process can beterminated or suspended temporarily at the low critical concentration ifit is desired to operate the reactor to sustain a chain reaction at thelow critical concentration. This latter procedure is efiicacious, forexample, in maximizing burn-up in a given quantity of slurry, inobtaining fissional products for industrial purposes, or the like.Following operation at the low critical concentration, if used, thechain reaction can be terminated simply by reducing the concentration ofthe slurry, as indicated by the isothermal curves of FIG. 3.

The slurry initially removed from the primary system by the slurryconcentrator 224i is approximately double the normal concentration andas the concentration in the primary system decreases so will that of theconcentrated output of the slurry concentrator 229. The heavy fractionof the slurry concentrator output is stored in the concentrated slurrytanks SZ'B to which the heavy fraction is conveyed by means of conduits252, 264, and "i6 and associated components as described previously.However, after the heavy fraction of the slurry concentrator 220 hasdecreased substantially below double the normal reactor concentrationthe slurry then is reconcentrated by conducting it from the drain tanks52B to the concentrator 22% and returning only the heavy fraction to thedrain tanks SZ'B. This is accomplished by conveying the slurry throughthe liquid header 110, conduits 130 and 128, pump bypassing conduit 354,conduit 124, and primary reactor system bypassing conduit 248 to theslurry concentrator inlet conduit 240. Then the heavy fraction isreturned via the concentrated slurry letdown device 262. The necessarydriving force is furnished by heat of radioactive decay, or if thelatter is insufficient by an over pressure supplied from the aforesaidexternal evaporation through conduits 317.

Alternatively, inasmuch as release of radioactive decay heat within theslurry stored in the drain tank's SZ'B will result in removal of excessslurry vehicle by vaporization, the slurry can be permitted to increaseto the desired consistency without removal from the drain tanks 5213.The vapors thus removed from the drain tanks are condensed in the draintank condensers 114 and 115 as described heretofore and the resultantliquid desirably is conveyed through the conduits 226, 228 and 230 tothe D 0 storage tanks 192. After the slurry has been concentrated toapproximately double the normal concentration within the drain tanks 5ZBand the concentration of slurry within the primary system has beenreduced to a corresponding concentration of about 30 grams per kilogramof D 0, the pairs of drain valves 106 are opened in the conduits 54 and104, and the slurry is conveyed to the repository tanks SZ'C via thedrain header 96 and associated conduits. This low concentration isselected in order to reduce erosion of the drain valves 1% insofar aspractical. More particularly, this concentration is selected because theslurry cannot be criticalized at any temperature at this concentration,as would be indicated by extrapolation of the temperature curves of FIG.3.

During the shutdown procedure decay heat is removed at a lowercontrolled rate from the primary system during the slurry dilutionprocess by reducing the water level at the steam side of the steamgenerators 32. Preheated feed water is employed for this purpose as inthe case of starting up the reactor to prevent thermal shock, and theresultant steam is condensed in the turbine condenser (not shown) or thelike. When the slurry concentration has been reduced in the primarysystem such that the average system temperature is in the neighborhoodof 250 F., heat removal by means of the steam generators should beterminated in order that the primary system will be maintained at thistemperature by the aforesaid heat of radioactive decay. At thistemperature the reactor can be restarted conveniently without imminentdanger of thermal shock. Moreover, the drain tank complex desirably ismaintained at this temperature, as described heretofore in connectionwith the start-up procedure, so that in the event of emergency drain theheated slurry conducted to the drain tanks SZC will not subject thesetanks to undue thermal stresses.

When the slurry concentration has been reduced to a sufficiently lowlevel that substantially no slurry particles will settle out in theprimary system, the primary pumps 36 are shut down, the heaters 7 0 ofthe pressurizing vessel 64 are deenergized, and the nine pairs of drainvalves 1% are opened, whereupon the contents of the primary system areemptied into the drain tanks 52'C. In this manner, thermal shock in thedrain tank complex will be practically nil and errosional damage to thedrain valves 1% will be minimized or eliminated altogether. The pressurein the drain tanks 52 resulting from decay heat is controlled by varyingthe cooling Water flow to the drain tank condensers 114- and 115. Themaximum pressure of the drain tanks, is arbitrarily set at 600 p.s.i.a.although the tanks are designed for 1509 p.s.i. This safety factor isdesirable inasmuch as available cooling water may be inadequate duringan emergency drain.

As indicated heretofore, the slurry concentration in each group of draintanks SZB and 52C is controlled by simply changing the flow path of thecondensed D 0 vapor from the drain tank condensers 114 and 115. Infurtherance of this purpose, a suitable concentration measuring device(not shown) is associated with each of the drain tanks 52.

When restarting the reactor the very dilute slurry contained in thedrain tank 52C is forced back into the primary reactor system bysuitable means. The very dilute slurry is withdrawn from the repositorydrain tanks 52K) through the liquid header and associated connectingconduits 112a and thence through conduits 130, 128 and 124 to one of thecirculating loops 25. In this example, the high head pumps 122 are ofrelatively low volumetric capacity, and therefore, are not employed forinitially filling the reactional vessel 2t) and associated circulatingloops 25. For this reason, the high head pumps are bypassed with avalved conduit 354, inasmuch as the initial reactor filling of verydilute slurry is incapable of supporting a chain reaction and can beintroduced at a rapid rate to save time in the restarting procedure.

In one arrangement the driving force required to transfer the diluteslurry from the drain tank complex to the primary reactor system issupplied by means of an overpressure applied in the form of D 0 steamfrom the afore mentioned external evaporator. The pressurizing D 0 steamis conveyed to the drain tanks SZC via the conduit 317, the drain header96, and associated connecting conduits 980, after opening appropriatevalves. The increased steam pressure, of course, forces the liquidcontained within the drain tank SZ'C through the conduits 1120, whichextend as aforesaid to bottoms of the tanks, and into the liquid header111 In another arrangement the driving force is supplied by heat ofradioactive decay of the slurry contained within the concentrated slurrytanks 52B. Since such material usually is relatively concentrated andcontains, after a period of circulation through the :reactional vessel26, a quantity of fissional products, a considerable amount of D 0 vaporis evolved from the tanks 52B by the decay heat of these products. Toemploy this vapor to force the contents of the repository drain tanks52'C back into the primary reactor system, valves 356 and 35% are closedin the conduits 152 and 171 connecting the entrainment separators 154and 172 and drain tank condensers 114 and 115, respectively, with thevapor header 144. At the same time, the valves 148 of the vapor headerare opened, and likewise valve 150, if there is an appreciable quantityof slurry in the slurry accumulator tank SZ'A; and the vapor issuingfrom the tanks 52B, and 52'A if any, is

conveyed through the vapor header 144 to the repository drain tanks52'C, via their individual connecting conduits 146a after opening valves335 therein. If this manner the vapor pressure built up in theconcentrated slurry tanks 52'B is applied to the top surfaces of theslurry contained within the dilute or repository drain tanks SZC.

As a result the dilute slurry is forced downwardly and out of the draintank 52C through the connecting conduits 112a and their extensions 142to the liquid header 110. The valve 134 in the liquid header having beenclosed, the dilute slurry then is caused to flow through the conduits130 and 128, the pump bypassing conduit 354, and the conduit 124 to theprimary reactor system. Inasmuch as the concentration of the slurrycontained within the repository tanks 52C is incapable of sustaining achain reaction, the primary system can be completely filled with thisdilute slurry as an initial step in restarting the reactor. Thereafter,the concentration of the slurry is increased in the manner describedheretofore in connection with initially starting the reactor. Thisarrangement also leaves the tanks 52C empty in the event of anycontingency or emergency during starting up or subsequent operation ofthe reactor.

An emergency drain procedure is followed in the event of equipmentleakage or failure in the primary reactor system. A rapid drainage ofthe reactor system obviates such conditions as extensive contaminationof the vapor container (not shown) surrounding the reactor plant,considerable loss of slurry and heavy water or other slurry vehicle, andcaking of slurry in the primary system due to failure of the primarypumps or the like. As shown in FIG. 2 of the drawings the primaryreactor system has a total of nine drain connections with one connection50 being made at the lower or inlet manifold of the reactor vessel(FIG. 1) and two connections being made at each of the circulating loops25. As described heretofore, each of these connections is coupledthrough conduits 54 or 1&4 in each of which are disposed a pair of stopvalves 106. This drainage system permits complete drainage from thelowest points of the entire primary reactor system. In this example, theentire contents of the primary system can be drained in approximatelyfifteen minutes.

If a maximum of radioactive decay heat is produced when the slurry isdrained in this fashion to the drain tanks 52, heat will be generatedtherein for a limited length of time at a greater rate than it can beremoved by the drain tank condensers 114 and 115. During the emergencydrain and for a short period thereafter, the temperature and pressure ofthe slurry in the drain tanks will rise to a maximum of about 545 F. and1000 p.s.i.a., in this arrangement.

The drain valves 196 can be operated manually, if desired, oralternatively, these valves can be operated automatically by suitablemechanisms (not shown), which mechanisms are actuated in turn by ahazardous abnormal condition such as an excessively high pressure in theprimary system, detection of radiation in the steam leaving one of thesteam generators 32, loss of power to the primary circulating pumps 36,leakage from the primary reactor system to the aforesaid vaporcontainer, or the like.

From the foregoing description, it Will be apparent that a novel andefficient method for operating a nuclear reactor has been disclosedherein. By causing the nuclear reactor to supply its own poison theconcentration of .the reactor fuel can be increased from the lowcritical concentration to the high critical concentration when startingup the reactor, and vice versa when shutting down the reactor, withoutundergoing the usual high temperature peak between these points as shownby the curves of FIG. 3 of the drawings and without adding externalpoison in the usual cumbersome manner to the primary reactor system.Although the invention is described in detail in connection with aslurry type homogeneous reactor system, it is to be understood and it isobvious that the method of operating the reactor disclosed herein can beadapted for use with any reactor system wherein the concentration offissile material therein can be varied between high and low criticalconcentrations.

" As a result, numerous modifications of the invention will occur tothose skilled in the art without departing from the spirit and scope ofthe present invention.

Accordingly, what is claimed as new is:

1. In a method for starting up a nuclear reactor system employingfissile material at least a portion of which can be circulated throughsaid system to vary the fuel concentration thereof, the steps comprisingincreasing the concentration of said fissile material from a relativelylower concentration to a low critical concentration at which saidfissile material is capable of sustaining a chain reaction, operatingsaid reactor at said low critical concentration until a predeterminedquantity of fi-ssional prodnets of said chain reaction are formed,terminating said operation for a period of two to ten hours to permitformation of a reactor poison from said fissional products, andincreasing the concentration of said fissile material in said reactor toa high critical concentration at which said fissile material is againcapable of sustaining a chain reaction.

2. In a method for starting up a nuclear reactor system employingfissile material at least a portion of which can be circulated throughsaid system to vary the fuel concentration thereof, the steps comprisingincreasing the concentration of said fissile material from a relativelylower concentration to a low critical concentration at which saidfissile material is capable of sustaining a chain reaction, operatingsaid reactor at said low critical concentration until a predeterminedquantity of fissional products of said chain reaction are formed,terminating said operation for a period of two to three hours to permitformation of a reactor poison from said fissional products, andincreasing the concentration of said fissile material in said reactor toa high critical concentration at which said fissile material is againcapable of sustaining a chain reaction.

3. In a method for starting up a nuclear reactor system employingfissile material at least a portion of which can be circulated throughsaid system to vary the fuel concentration thereof, the steps comprisingincreasing the concentration of said fissile material from a relativelylower concentration to a low critical concentration at which saidfissile material is capable of sustaining a chain reaction, operatingsaid reactor at said low critical concentration for about two hours toform a quantity of fissional products of said chain reaction,terminating said operation for a period of two to ten hours to permitfor mation of a reactor poison from said fissional products, andincreasing the concentration of said fissile material in said reactor toa high critical concentration at which said fissile material is againcapable of sustaining a chain reaction.

4. In a method for operating a nuclear reactor system employing fissilefuel material at least a portion of which can be circulated through saidsystem to vary the fuel concentration thereof, the steps comprisingoperating said reactor with a fuel concentration corresponding to afirst critical concentration at which said fissile material is capableof sustaining a chain reaction until a predetermined quantity offissional products of said chain reaction are formed, terminating saidoperation for a period of two to ten hours to permit formation of areactor poison from said fissional products, and changing theconcentration of said fuel material to a second critical concentrationat which said fissile material is capable of sustaining a chainreaction.

5. In a method for operating a nuclear reactor system employing fissilefuel material at least a portion of which can be circulated through saidsystem to vary the fuel concentration thereof, the steps comprisingoperating said reactor with a fuel concentration corresponding to afirst critical concentration at which said fissile material is capableof sustaining a chain reaction for a period of about two hours to form aquantity of fissional products of said chain reaction, terminating saidoperation for a period of two to ten hours to permit formation of areactor poison from said fissional products, and changing theconcentration of said fuel material to a second critical concentrationat which said fissile material is capable of sustaining a chainreaction.

6. In a method for operating a nuclear reactor system employing fissilefuel material at least a portion of which can be circulated through saidsystem to vary the fuel concentration thereof, the steps comprisingoperating said reactor system with a fuel concentration corresponding toa first critical concentration at which said fissile material is capableof sustaining a chain reaction for a period sufficient to form aquantity of fissional products of said chain reaction and at a givenoperating temperature, terminating said chain reaction by permittingsaid operating temperature to rise slightly, maintaining said reactorsystem in a sub-critical condition for a predetermined period to permitformation of a reactor poison from said fissional products, and changingthe concentration of said fuel material while said reactor system ismaintained in a sub-critical condition by said reactor poison to asecond critical concentration at which said fissile material is capableof sustaining a chain reaction.

7. In a method for operating a nuclear reactor system employing fissilematerial at least a portion of which can be circulated through saidsystem to vary the fuel concentration thereof, the steps comprisingoperating said reactor with a fuel concentration corresponding to afirst critical concentration at which said fissile material is capableof sustaining a chain reaction for a period sufiicient to form aquantity of fissional products of said chain reaction and at a givenoperating temperature, increasing said temperature to terminate saidoperation for a period sufficient to permit formation of a reactorpoison from said fissional products, maintaining the heat of radioactivedecay of said fissional products within said reactor system to maintainsaid last-mentioned temperature unti-l a sufficient quantity of saidreactor poison can be formed to maintain said reactor system in asub-critical condition, and changing the concentration of said fuelmaterial while said reactor system is maintained in said sub-criticalcondition to a second critical concentration at which said fissilematerial is capable of sustaining a chain reaction.

8. In a method for operating a nuclear reactor system employing fissilefuel material at least a portion of which can be circulated through saidreactor system to vary the fuel concentration thereof through first andsecond critical concentrations and an intermediate range ofsuper-critical concentrations relative to a given operating temperature,the steps comprising operating said reactor system with a fuelconcentration corresponding to said first critical concentration for apredeterminable period sufficient to form a quantity of fissionalproducts in said reactor system, terminating said operation for apredeterminable period sufficient to permit formation of a reactorpoison from said fissional products, and changing the concentration ofsaid fuel material through said super-critical concentrations while saidreactor system is maintained in a sub-critical condition by said reactorpoison to said second critical concentration.

References Cited in the file of this patent UNITED STATES PATENTS2,743,225 Ohlinger Apr. 24, 1956 2,904,488 Thamer et a1. Sept. 15, 19592,928,779 Weills et a1. Mar. 15, 1960 2,938,844 Graham et al. May 31,1960 2,945,795 Winters et a1. July 19, 1960 OTHER REFERENCES Principlesof Nuclear Reactor Engineering by Samuel Glasstone, D. Van Nostrand Co.,New York, 1955, pp. 754-758; 352, 353, 364-368; 326, 327, 266-270;183-189.

Proceedings of the International Conference on the Peaceful Uses ofAtomic Energy, vol. 3, United Nations, N.Y., 1955, pp. -187, 263-282.

Westinghouse Engineer 7 (No. 2, March 1957), pp. 34-39.

Nuclear Power, May 1957, pp. 193-195.

PNG-7 US. Atomic Engery Commission Aqueous Homogeneous Reactors,February 1956, pp. 1-19.

1. IN A METHOD FOR STARTING UP A NUCLEAR REACTOR SYSTEM EMPLOYINGFISSILE MATERIAL AT LEAST A PORTION OF WHICH CAN BE CIRCULATED THROUGHSAID SYSTEM TO VARY THE FUEL CONCENTRATION THEREOF, THE STEPS COMPRISINGINCREASING THE CONCENTRATION OF SAID FISSILE MATERIAL FROM A RELATIVELYLOWER CONCENTRATION TO A LOW CRITICAL CONCENTRATION AT WHICH SAIDFISSILE MATERIAL IS CAPABLE OF SUSTAINING A CHAIN REACTION, OPERATINGSAID REACTOR AT SAID LOW CRITICAL CONCENTRATION UNTIL A PREDETERMINEDQUANTITY OF FISSIONAL PRODUCTS OF SAID CHAIN REACTION ARE FORMED,TERMINATING SAID OPERATION FOR A PERIOD OF TWO TO TEN HOURS TO PERMITFORMATION OF A REACTOR POISON FROM SAID FISSIONAL PRODUCTS, ANDINCREASING THE CONCENTRATION OF SAID FISSILE MATERIAL IN SAID REACTOR TOA HIGH CRITICAL CONCENTRATION AT WHICH SAID FISSILE MATERIAL IS AGAINCAPABLE OF SUSTAINING A CHAIN REACTION.